1. Field of the Invention
The present invention relates generally to mirrors with aperture holes, and more particularly to methods of fabricating the mirrors with extremely small aperture holes that are sharp and cleanly defined.
2. Description of the Prior Art
The heads in many high density disk drives actually fly above the magnetic surface of the disk media. The "flying height" is a critical parameter and must be carefully and accurately measured to assure optimum performance.
Photo Research.RTM., a division of Kollmorgen (Chatsworth, CA), has made commercially available a family of fast scanning PC-controlled spectroradiometers (e.g., model PR-704/714) that are useful in magnetic head flying height measurements. An incoming source of light passes through an entrance slit and falls on a wavelength disperser, or diffraction grating. (See, Photo Research product bulletin no. 695, copyright 1990.) A multi-element photodetector is placed in the focal plane of the diffracted spectrum. Each narrow band wavelength is sensed by a separate photo element. Thus the entire spectrum is electronically scanned. The Photo Research system is described as being compatible with the AT-bus for the IBM personal computer and has software that loads on the PC. A feature that is included is labeled Automatic Adaptive Sensitivity. (See, U.S. Pat. No. 4,554,460.)
A technique for measuring the very small spaces such as between a glass disk and magnetic head or slider, is described in U.S. Pat. No. 4,593,368, issued Jun. 3, 1986, to Fridge, et al. A broad spectrum light is reflected off of two non-contacting surfaces and the reflected light combines constructively and destructively such that the amplitude varies with wavelength. The combined signals are analyzed at "a large number of wavelengths", and the wavelength or wavelengths of maximum intensity and/or minimum intensity are identified. Typically, the light intensity minima is said by Fridge, et al., to occur at d=(1/2)nL and maxima at d=(1/2)(n-1/2), where d is the separation, L is the wavelength, and n is a positive integer indicating the order. The actual plot of intensity versus wavelength is correlated with the theoretical intensity versus wavelength function, and this correlation of curve fitting is carried forward to calculate the spacing "d" to any desired accuracy within the limits of the input data. Fridge, et al., lists one useful relationship as being: ##EQU1## where L.sub.1 and L.sub.2 are maximum and minimum intensity wavelengths, respectively.
Prior art methods of manufacturing mirrors with small aperture holes (about five mils) have proved difficult to maintain in production volumes and low yields from the limited number of manufacturers in the world has been a critical path element in spectroradiometer production for those units requiring small apertures. Etching methods that have been used result in holes that are non-cylindrical and lack good sharp corners. A black, non-reflecting irregular zone on the mirror surface forms around the aperture hole and appears to a camera focussed on the reflection to be the aperture hole, when, in fact, the actual aperture hole is smaller and located somewhere within the zone. These prior art processes also tend to produce related black spots, or even holes, all over the front side mirror surface. As the state of the art progresses in spectroradiometric applications, the sizes of the required aperture holes decreases, and the previously acceptable surface defects can become easily confused with the intended aperture hole during measurements.